A magnetostriction phenomenon has been known in which a material changes its form in a magnetic field given from an external unit. The material that develops the supermagnetostriction phenomenon is called supermagnetostriction material. A new supermagnetostriction material has been developed in recent years producing a dimensional change of 1000 ppm or more in a state where a magnetic field is applied to the supermagnetostriction material from the external unit.
It has further been known that the supermagnetostriction materials generate a large stress, and some of them attain 400 kgf/mm2 or more. Further, the supermagnetostriction material quickly responds when it comes to a change in the shape of the material for a magnetic field from the external unit, and some of them change the size in less than one microsecond after the magnetic field is applied.
It has been attempted to form a supermagnetostriction material of this type as a rod-like supermagnetostriction element and to use the dimensional change produced by the supermagnetostriction element as an actuator. It has further been attempted to use the actuator based on the magnetostriction element as a drive source for a speaker.
The following patent document discloses an art of using, as a speaker, a rod-like supermagnetostriction element obtained by using the supermagnetostriction material.    Patent document 1: JP-A-10-145892 (page 1, FIG. 1)